[Eng-div] Fwd: Re: [Accelerator_staff] Accelerator Seminar: Binping Xiao - 5/17/12

[Gayle Coleman]

REMINDER!

On 5/14/2012 9:25 AM, Audrey N. Barron wrote:
> ACCELERATOR SEMINAR
>
> "Surface Impedance of Superconducting Radio Frequency (SRF) Materials"
>
> Binping Xiao
> College of William Mary and Jefferson Lab
>
> Superconducting radio frequency (SRF) technology is widely adopted in 
> particle accelerators. There remain many open questions, however, in 
> developing a systematic understanding of the fundamental behavior of 
> SRF materials, including niobium treated in different ways and various 
> other bulk/thin film materials that are fabricated with different 
> methods under assorted conditions. A facility that can measure the SRF 
> properties of small samples in a range of 2~40 K temperature is needed 
> in order to fully answer these questions. The Jefferson Lab surface 
> impedance characterization (SIC) system has been designed to attempt 
> to meet this requirement. It consists of a sapphire-loaded cylindrical 
> Nb TE_011 cavity at 7.4 GHz with a 50 mm diameter flat sample placed 
> on a non-contacting end plate and uses a calorimetric technique to 
> measure the radio frequency (RF) induced heat on the sample. Driving 
> the resonance to a known field on this surface enables one to derive 
> the surface resistance of a relatively small localized area. Tests 
> with polycrystalline and large grain bulk Nb samples have been done at 
> <15 mT magnetic field. Based on BCS surface impedance, least-squares 
> fittings have been done using SuperFit2.0, a code developed by G. 
> Ciovati and the author.
>
> Microstructure analyses and SRF measurements of large scale epitaxial 
> MgB2 films have been reported. MgB2 films on 5 cm dia. sapphire disks 
> were fabricated by a Hybrid Physical Chemical Vapor Deposition (HPCVD) 
> technique. The electron-beam backscattering diffraction (EBSD) results 
> suggest that the film is a single crystal complying with a 
> MgB2(0001)//Al2O3(0001) epitaxial relationship. The SRF properties of 
> different film thicknesses (200 nm and 350 nm) were evaluated using 
> SIC system under different temperatures and applied fields at 7.4 GHz. 
> A surface resistance of 9±2 μΩ has been observed at 2.2 K.
>
> Based on BCS theory with moving Cooper pairs, the electron states 
> distribution at 0K and the probability of electron occupation with 
> finite temperature have been derived and applied to anomalous skin 
> effect theory to obtain the surface impedance of a superconductor with 
> moving Cooper pairs. We present the numerical results for Nb.
>
> Thursday, May 17, 2012
> 11:00 a.m.
> CEBAF Center, Room F326/327
>
> Â 
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